The long-term objective of this study is to derive general principles for the functioning of local circuit neurons, especially those cells which have synaptic output from their integrating regions (e.g., dendro-dendritic synapses) and which may use graded synaptic transmission below the threshold for spikes. The immediate objective is to understand the functioning of an invertebrate system, the stomatogastric ganglion of the crab Cancer borealis. The focus of this study will be experiments designed to elucidate the role of calcium in stomatogastric cell function. Questions will include: 1) When is voltage-dependent calcium influx due mostly to graded voltage changes (the pyloric rhythm) and when is it due mostly to spikes? Does calcium influx vary with the cell type, the location within the neuron, or with synaptic or neuromodulator activity? 2) How do neuromodulators affect the resting internal calcium concentration and the voltage- dependent calcium influx in different regions of identified cells? Methods to be used in this study include: intracellular somatic and neuropil recording, simultaneous measurements of internal calcium fluctuations in different regions of single neurons by use of the calcium indicator dye Arsenazo III and a photodiode array, and measurements of resting internal calcium concentrations with Fura 2 and CCD camera. Of necessity, most studies of neuron function have been constrained spatially: intracellular electrodes are placed in only one or two simultaneous locations in single cells, typically the soma. This study will provide an opportunity to look at the behavior of an entire neuron during its normal range of behaviors as it uses what might be considered its most interesting ion, calcium.